Generally, a trimmer employs a drive mechanism for transferring the rotation of an internal combustion engine through a centrifugal clutch. For example, Japanese Patent Laid-open Publication No. Hei 11-196648 discloses a portable trimmer comprising a centrifugal clutch provided with a brake mechanism for preventing a cutter blade from unintentionally rotating when an operator discontinues a trimming operation. This brake mechanism includes a brake band disposed along the outer peripheral surface of a clutch drum of the centrifugal clutch. During the initial start-up of the internal combustion engine, the brake band is located at a clamp position where it is pressed against the outer peripheral surface so as to tighten the outer peripheral surface to brake the rotation of the clutch drum. A brake control lever can be operated to move the brake band to a release position where the tightening force of the brake band is released to allow the clutch drum to be rotated so that the cutter blade is rotatably driven.
More specifically, the above brake mechanism comprises the brake band disposed along the outer peripheral surface of the clutch drum, and releasing means for releasing the brake band. The releasing means includes a swingable member biased toward its initial position by a spring, and a control wire having one end connected to the swingable member and the other end connected operationally to the brake control lever. The brake band has one end fixed to a housing and the other end connected to the swingable member. The brake control lever can be operated to pull the swingable member against the spring force of the spring or swing the swingable member from the initial position to its swing position so as to move the brake band from the tightened position to the release position.
In addition to the above patent publication, various brake mechanisms of the centrifugal clutch are disclosed, for example, in Japanese Patent Laid-open Publication No. Sho 51-63222, Japanese Patent Publication No. Sho 52-12089, Japanese Utility Model Publication No. Sho 54-1374 and Japanese Patent Laid-open Publication No. Sho 54-40954. These brake mechanisms commonly include a brake lining attached over an effective contact length of the inner surface of a brake band on its entirety. Further, Japanese Patent Laid-open Publication No. 2002-176822 discloses another type brake mechanism including a pair of swingable brake members for pressing the above clutch drum from both sides thereof, as a substitute for a brake band.
However, taking the brake mechanism disclosed in the aforementioned Japanese Patent Laid-Open Publication No. Hei 11-196648 as an example, the outer periphery of the clutch drum can be undesirably worn out in an early stage depending on its use conditions, because a braking force for the clutch drum is obtained by tightening its outer peripheral surface with the brake band. In addition, the brake mechanisms using a brake band generally involve a problem of abnormal noises.
For example, as shown in FIG. 1, a conventional trimmer 2 comprises an internal combustion engine 4 of a small air-cooled 2-cycle or 4-cycle gasoline engine, an operation rod 6 extending forwardly from the engine 4, a cutting blade 8 attached to the front end of the operation rod 6 through a gear case 7, and a handle 10 with a brake control lever 20 attached to an intermediate region of the operation rod 6. The trimmer 2 includes a driveline comprised of a centrifugal clutch (not shown) for transferring the rotation of the engine 4, an output shaft 14 extending from the centrifugal clutch along the inner space of the operation rod 6, and the cutting blade 8 connected to the output shaft 14 through the gear case 7.
Given that the centrifugal clutch is provided with the aforementioned brake mechanism having the brake band, upon the release of the brake band, the power of the engine 4 is transferred to the cutter blade 8 through the driveline to rotate the cutter blade 8. When the brake control lever 20 is released to discontinue a trimming operation, the outer peripheral surface of the clutch drum is tightened and braked by the brake band to stop the rotation of the output shaft 14, while the cutter blade 8 tends to keep rotating according to inertia force. As a result, an abnormal noise will be generated from the bear box 7 interposed between the stopped output shaft 14 and the cutter blade 8 which is continuously rotating.
While the rotation of the clutch drum is usually stopped by a tightening force of the brake band, deficiency in the clamping force will result in slippage of the clutch drum and allow the clutch drum to rotate, which leads to difficulty in eliminating the idle running of the cutter blade within a given time. On the other hand, excess in the tightening force will cause too hasty stop of the clutch drum, which leads to unendurable reaction force against operator's hands, abnormal noises due to interference between gears provided in the driveline, damage in associated components, or looseness in the fixing of the cutting blade. Thus, it is desired to set the tightening force of the brake band against the clutch drum at an adequate value.